This application is a U.S. national phase application of PCT international application PCT/JP00/00498.
The present invention relates to a non-aqueous electrolyte secondary battery (cell), and more particularly to a negative electrode material used therein.
Generally, a lithium secondary battery has a high electromotive force and a high energy density. Lithium secondary batteries are used in portable information terminal, mobile communication appliance, portable electronic devices, household small power storage unit, motorcycle, electric vehicle, hybrid electric car, etc.
A lithium secondary battery using lithium metal as negative electrode material has a high energy density. When charging, however, dendrite deposits into the negative electrode, and as charging and discharging are repeated, the dendrites breaks through the separator to reach the positive electrode side, thereby causing internal short circuit. Besides, the depositing dendrite has a large specific surface area, and is high in reaction activity. Therefore, the dendrite depositing on the surface of the negative electrode reacts with the solvent in the electrolyte solution, and an interface film like a solid electrolyte lacking in electron conductivity is formed on the surface of the negative electrode. As a result, the internal resistance of the battery is raised, or particles isolated from the network of electron conduction are formed. These factors cause to lower the charging and discharging efficiency.
As a negative electrode material replacing the lithium metal, a carbon material capable of occluding and emitting lithium ions is used. Usually, metal lithium does not deposit on the carbon material negative electrode. Hence, internal short circuit by dendrite does not take place. However, the theoretical capacity of graphite which is one of the carbon materials is 372 mAh/g, and this theoretical capacity is about one-tenth of the theoretical capacity of Li metal alone.
As other negative electrode material, a single metal material or a single nonmetal material capable of forming a compound with lithium is known. For example, among compounds of silicon (Si) and lithium, the composition of the compound having the highest lithium content is Li22Si5. In this range, usually, metal lithium does not deposit. Therefore, internal short circuit by dendrite does not occur. The electrochemical capacity of these compounds and various single materials is 4199 mAh/g, and this capacity is larger than the theoretical capacity of graphite.
As other compound negative electrode material, a nonferrous metal silicide composed of transition element is disclosed in Japanese Laid-open Patent No. 7-240201. A negative electrode material made of an inter-metallic compound containing at least one of 4B group elements, and P, and Sb, of which crystal structure is any one of CaF2 type, ZnS type, and AlLiSi type is disclosed in Japanese Laid-open Patent No. 9-63651. Further, inter-metallic compounds expressed by AB2 type are disclosed in Japanese Laid-open Patent No. 10-312804 and Japanese Laid-open Patent No. 10-302770.
Still more, inter-metallic compounds based on Si or Sn, and alloys and other similar negative electrode materials are disclosed in Japanese Laid-open Patent No. 10-162823, Japanese Laid-open Patent No. 10-294112, Japanese Laid-open Patent No. 10-302770, and Japanese Laid-open Patent No. 10-312804.
However, these negative electrode materials having a higher capacity than the carbon material have the following problems.
Negative electrode materials such as single metal materials and single nonmetal materials capable of forming compounds with lithium are commonly inferior in charging and discharging cycle characteristic as compared with carbon negative electrode materials. The reason is not clear, but is estimated as follows.
For example, silicon contains eight silicon atoms in its crystallographic unit lattice (cubic system, space group Fd-3m). Converting from its lattice constant a=0.5420 nm, the unit lattice volume is 0.1592 nm3, and the volume per one silicon atom is 19.9xc3x9710xe2x88x923 nm3. Judging from the two-element phase diagram of silicon and lithium, it is estimated that two phases of silicon and compound Li12Si7 coexist in the initial stage of reaction of electrochemical compound formation between silicon and lithium at room temperature. A crystallographic unit lattice (orthorhombic system, space group Pnma) of Li12Si7 contains 56 silicon atoms. Converting from its lattice constant a=0.8610 nm, b=1.9737 nm, c=1.4341 nm, the unit lattice volume is 2.4372 nm3, and the volume per one silicon atom is 43.5xc3x971031 3 nm3. The volume per one silicon atom is calculated by dividing the unit lattice volume by the number of silicon atoms in the unit lattice. According to calculation from this value, when silicon reacts to form a compound Li22Si7, the material volume expands by 2.19 times. By reaction in two-phase coexistent state of silicon and compound Li12Si7, since silicon is partly transformed into the compound Li12Si7, their volume difference increases, the material is largely distorted, and cracks are likely to occur and the particles become finer. Further, when an electrochemical reaction to form compound of lithium and silicon is promoted, finally, the compound Li22Si5 having the highest lithium content is formed. A crystallographic unit lattice (cubic system, space group F23) of Li22Si5 contains 80 silicon atoms. Converting from its lattice constant a=1.8750 nm, the unit lattice volume is 6.5918 nm3, and the volume per one silicon atom is 82.4xc3x9710xe2x88x923 nm3. This value is 4.14 times that of single silicon, and the material is largely expanded. In discharge reaction of negative electrode material, lithium is decreased in the compound, and the material contracts in the process of this reaction. Thus, since the volume difference is significant between charging time and discharging time, a large distortion occurs in the material, which is regarded as the cause of cracking and pulverization of particles. Further, a space is formed in the finer particles, and the electron conduction network is broken, and the portion not engaged in the electrochemical reaction increases, thereby lowering the charging and discharging capacity.
That is, the reason of poor charging and discharging cycle characteristic of the single metal material and single nonmetal material capable of forming a compound with lithium, as compared with the carbon negative electrode material, is estimated to be the volume change and the texture change caused by such volume change.
On the other hand, negative electrode materials made of an inter-metallic compound containing at least one of silicide such as nonferrous metal of transition element, 4B group elements, P, and Sb, of which crystal structure is any one of CaF2 type, ZnS type, and AlLiSi type is disclosed in Japanese Laid-open Patent No. 7-240201 and Japanese Laid-open Patent No. 9-63651.
The battery using the silicide negative electrode material of nonferrous metal composed of transition element disclosed in Japanese Laid-open Patent No. 7-240201 has an improved charging and discharging cycle characteristic as compared with the battery using lithium metal negative electrode material. It is estimated from the battery capacity measured after 1 cycle, 50 cycles and 100 cycles in the embodiment and comparative example described in the prior art. However, this battery using the silicide negative electrode material is increased in the battery capacity by only about 12% at maximum as compared with the battery using natural graphite negative electrode material. That is, although not clearly stated in the specification of the prior art, the silicide negative electrode material of nonferrous metal composed of transition element does not seem to increase the capacity significantly as compared with the graphite negative electrode material.
The battery using the negative electrode material disclosed in Japanese Laid-open Patent No. 9-63651 has an improved charging and discharging cycle characteristic as compared with the battery using Li-Pb alloy negative electrode material, and has a higher capacity than the battery using the graphite negative electrode material. However, in a range of 10 to 20 charging and discharging cycles, the discharge capacity is decreased significantly. In the battery Using Mg2Sn which is estimated to be most favorable, the discharge capacity is decreased to about 70% of the initial capacity in about 20 cycles.
Materials disclosed in Japanese Laid-open Patent No. 10-162823 and Japanese Laid-open Patent No. 10-294112 are materials containing Sn or Si, in which Sn or Si is contained by more than about 50 atom % of the whole material. Indeed, a battery of a higher capacity is obtained by increasing the content of Sn or Sn in the material. Further, the cycle characteristic is slightly improved as compared with the material using Sn or Si alone. However, in the battery containing by more than about 50 atom % of Sn or Si, when charging and discharging are repeated more than 100 cycles, the cycle deterioration of the battery characteristic is emphasized by expansion and shrinkage of material occurring in every cycle.
Materials disclosed in Japanese Laid-open Patent No. 10-302770 and Japanese Laid-open Patent No. 10-312804 are AB2 type inter-metallic compounds, and have an excellent coulomb efficiency and rate characteristic. In the battery using these materials, the initial discharge capacity is larger as compared with the battery using the conventional graphite. However, the discharge capacity in 10 cycles of the battery using these materials is already lowered to the capacity maintenance rate of 90% order, and these materials do not have excellent cycle characteristic.
A non-aqueous electrolyte secondary battery (cell) of the invention comprises a negative electrode, a positive electrode, and a non-aqueous electrolyte, in which the negative electrode contains an alloy having Si, a first element and a second element, the first element includes at least one element selected from the group consisting of a second group element except Mg in the periodic table, transition elements, a twelfth group element, a thirteenth group element except B, and a fourteenth group element except Si, and the second element includes at least one element of B and Mg.
A negative electrode material for non-aqueous electrolyte secondary battery of the invention comprises Si, a first element, and a second element, in which the first element includes at least one element selected from the group consisting of a second group element except Mg in the periodic table, transition elements, a twelfth group element, a thirteenth group element except B, and a fourteenth group element except Si, and the second element includes at least one element of B and Mg.
Preferably, the first element is at least one element of Ni and Co.
Preferably, the second element is contained in a range of 0.1 part by weight to 30 parts by weight in 100 parts by weight of the sum of weight of Si and first element.
In this composition, a secondary battery having a high capacity and an excellent charging and discharging cycle characteristic is obtained.